1. Field of the Invention
The present invention relates to the deposition of transition metal carbide thin films. More specifically, the present invention relates to the use of sequential self-saturating surface reactions to form transition metal carbides on various substrates.
2. Description of the Related Art
Carbides of transition metal elements in groups 4 (Ti, Zr, Hf), 5 (V, Nb, Ta) and 6 (Cr, Mo, W) of the periodic table possess several attractive properties. They are relatively inert, have very high melting points, are extremely hard and wear resistant, and have high thermal conductivity and metal-like electrical conductivity. For these reasons, transition metal carbides have been proposed for use as low resistance diffusion barriers in semiconductor fabrication (see, e.g., international patent application WO 00/01006; U.S. Pat. No. 5,916,365).
General information about metal carbides can be found, for example, in Ullmann""s Encyclopedia of Industrial Chemistry, 5th Edition, Vol. A5, VCH Verlagsgesellschaft, 1986, pp. 61-77, and in the Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, Vol. 4, John Wiley and Sons, Inc., 1992, pp. 841-878. Transition metal carbides can have a wide range of compositions. Ordered and disordered carbon deficient forms exist, of which the tungsten carbides W3C, W2C, WC and WC1-x are examples. In these forms, carbon resides in the interstitial cavities between metal atoms.
Suggested deposition methods include Chemical Vapor Deposition (CVD), Metal Organic Chemical Vapor Deposition (MOCVD) and Physical Vapor Deposition (PVD).
Carbides have been deposited by CVD type processes wherein more than one source chemical is present in the reaction space at the same time. A CVD method of depositing tungsten carbide from tungsten hexafluoride, hydrogen and a carbon-containing gas has been described, for example, in international patent application WO 00/47796. The carbon-containing gas is initially thermally activated. All of the gaseous source chemicals are present at the same time in the reaction space, resulting in the deposition of nonvolatile tungsten carbide on the substrate. A CVD reaction of WF6 with trimethylamine and H2 has been disclosed for yielding WC films at 700xc2x0 C.-800xc2x0 C. and beta-WC1-x films at 400xc2x0 C.-600xc2x0 C. (Nakajima et al., J. Electrochem. Soc. 144:2096-2100 (1997)). The H2 flow rate affects the deposition rate of tungsten carbide. One problem with the disclosed process is that the substrate temperature is rather high relative to thermal budgets for state-of-the-art semiconductor fabrication, particularly in metallization stages.
MOCVD processes utilize organometallic compounds that are thermally decomposed on the substrate or combined with other organic compounds in gas phase and then contacted with the substrate thus breaking the source chemical molecules and forming the final product. Tungsten carbide has also been deposited on substrates by the thermal decomposition of organotungsten derivatives of W(CO)6 at low pressures (Lai et al., Chem. Mater. 7:2284-2292 (1995)). Similarly, TiC has been deposited in a CVD process by the thermal decomposition of organometallic titanium compounds (Girolami et al., Mater. Res. Soc. Symp. Proc. 121:429-438 (1988)). U.S. Pat. No. 5,916,365 also discloses thermal decomposition of pentadimethyl-aminotantalum. In these processes, the source chemical molecules contain both the metal and the carbon. However it""s utility on complex, irregular surfaces is not known.
PVD processes generally deposit along a line-of-sight. One method of depositing tantalum carbide for a diffusion barrier layer by PVD has been described in U.S. Pat. No. 5,973,400. The tantalum carbide layer was formed by sputtering tantalum or tantalum carbide under N2/CH4/Ar atmosphere. Line of sight deposition, however, means that complex substrate contours will have insufficient thin film coverage in shaded areas. Additionally, line-of-sight deposition means that low-volatility source material arriving directly from the source to the substrate will likely adhere to the first solid surface that it encounters, thus producing low-conformality coverage.
Thus, there is a need in the art for improvements in methods of depositing transition metal carbides.
In accordance with one aspect of the invention, a method is disclosed for depositing a transition metal carbide thin film by an atomic layer deposition (ALD) process. In the illustrated embodiment, vapor-phase pulses of at least one transition metal source compound and at least one carbon source compound are alternately fed into a reaction space containing a substrate.
The transition metal source compound preferably comprises a metal source gas selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W. An exemplary transition metal source gas is a metal halide, such as tungsten hexafluoride. Exemplary carbon source compounds include boron compounds, silicon compounds and phosphorous compounds. Desirably, in these exemplary source gas compounds, either boron, silicon or phosphorus bond directly to carbon.
The process is of particularly utility when depositing ultrathin, high quality layers, such as typically demanded in the field of semiconductor fabrication. For example, a metal carbide thin film can advantageously form thin diffusion barrier that is conductive and conformal over integrated circuit topography (e.g., dual damascene trenches and vias).